The present invention relates to a magnetic sensor for detecting magnetic fields, more specifically to a magnetic sensor which is suitably used in a magnetic read head of a magnetic recording apparatus.
With the recent high densities of the magnetic recording technique, relative speeds between magnetic recording media and read heads have much decreased. The conventional induction type read heads have found it difficult to have sufficient reading signals.
Then, in order that high reading signals are available at even the low relative speed, anisotropic magnetoresistive (AMR) effect magnetic heads for detecting magnetic fields themselves are developed. The anisotropic magnetoresistive effect is a phenomena that an electric resistance changes by some percentages corresponding to magnetized states of a magnetic substance. As materials of the anisotropic magnetoresistive effect magnetic heads (AMR magnetic heads), NiFe alloy (permalloy) is dominantly used.
However, AMR magnetic heads can have some percentage changes in the reading signals, and magnetic sensors having reading output changes of higher percentages. As such magnetic sensors are proposed giant magnetoresistive (GMR) effect magnetic sensors using giant magnetoresistive effect, ferromagnetic tunnel junction magnetic sensor using ferromagnetic tunnel junction, and others.
A ferromagnetic tunnel junction magnetic sensor includes a multi-layer body of a ferromagnetic layer/an insulating layer/ferromagnetic layer in which the insulating layer is sandwiched between the ferromagnetic layers. When a voltage is applied between the ferromagnetic layers to tunnel electrons, a tunneling probability of electrons changes depending on a relative angle of both ferromagnetic layers to a magnetization direction. This is because electron spin of one of the ferromagnetic layers, which supplies electrons is polarized, and electrons tunnel in the polarized state.
A change of the tunneling probability is given by a product of polarizabilities of both ferromagnetic layers. That is, a difference .DELTA.R of a total resistance R between a maximum value and a minimum value is expressed by EQU .DELTA.R/R=2.times.P1.times.P2.
Accordingly, in the ferromagnetic tunnel junction magnetic sensor, one of the ferromagnetic layer is formed of a material having a larger coercive force, and the other ferromagnetic layer is formed of a material of a smaller coercive force. When an external magnetic field changes, a magnetization direction of the ferromagnetic layer of the larger coercive force does not change, but a magnetization direction of the ferromagnetic layer of the smaller coercive force changes to agree to a direction of the external magnetic field. As a result, when a magnetization direction of an external magnetic field changes, a relative angle between magnetization directions of both ferromagnetic layers changes, whereby a tunnel current changes, and the external magnetic field can be detected.
As described above, in the above-described ferromagnetic tunnel junction magnetic sensor it is necessary that one of the ferromagnetic layers is formed of a material having a large coercive force so that a magnetization direction does not change even when an external magnetic field changes. Accordingly, to use the ferromagnetic tunnel junction magnetic sensor as a magnetic read head, a material of the ferromagnetic layer is limited to a material, such as Fe, Co, Ni or others, a magnetization direction of which does not change. However, the polarizabilities of these materials are 10-40%, and in principle changes of tunneling probabilities have upper limits. The coercive forces of these materials are tens to hundreds Oe, and when a magnetic field of a higher than such coercive force is applied due to a cause, characteristics are adversely changed.
An object of the present invention is to provide a magnetic sensor which can overcome the above-described disadvantages, and has a high magnetoresistance ratio and is hard against disturbing magnetic fields.